1. Technical Field of the Invention
This invention relates to a mechanism for positioning an output member of a mechanical system driven by a step motor under the operation of a control system. Step motors controlled in accordance with the invention are capable of driving a mechanical system where backlash is a substantial percentage of a single step. The control system of this invention is implemented in microcode for maximum flexibility.
2. Description of the Prior Art
Step motors are widely used in the control of mechanical systems; and, in particular, mechanical systems that are used in association with information handling equipment. A step motor is particularly suited for such use because it is a digital device; movement occurs in incremental steps based upon discrete input pulses. Examples of step motor applications are disk and tape drives, computer printers and robot manipulators. These applications generally incorporate a step motor as a drive means for a mechanical system that requires very precise and accurate movement. This precision and accuracy is achieved by the small steps the step motor can make. A limitation occurs when a single step of the step motor is of the same order of magnitude as the backlash in a mechanical system. A continuing problem facing this art is the design of suitable control systems that both optimize the step motor performance and provide consistent and accurate response by the mechanical system being driven.
In a step motor driven mechanical system there is an intermediate mechanical element such as a transmission connecting a step motor to an output member. For example, the mechanical system includes a plurality of gears that mesh with one another. In such a device, the tooth of one gear interacts with the inter-tooth space of a second gear to impart motion to that second gear. Due to manufacturing tolerances, the distance between adjacent gear teeth, or inter-tooth space, is greater than the gear tooth with which it is to mesh. When the step motor and drive gear stop abruptly, the driven gear will continue to move until one of the driven gear teeth collide with the drive gear tooth causing the driven gear to oscillate and the system to recoil. This is known as backlash and is an especially difficult problem to control in any application where a single step of the step motor is of the same order of magnitude as the backlash in the system.
Various solutions have been proposed for this recurring problem. Those solutions include mechanical damping, deceleration ramping or controlling the timing between steps, electronic braking of the motor and, finally, monitoring the output position of the system to control the input signal. All four methods have significant disadvantages. Mechanical systems such as the helix screw and anti-backlash nut are complicated and costly to implement. In addition, such a mechanical system is generally designed for a specific device and cannot be used for other applications. Deceleration ramping is an adequate solution if the particular application always calls for a plurality of steps, however, for many applications, this technique is inadequate for driving the step motor a single step. In electronic braking, current is allowed to flow temporarily in the inactivated coils to apply a counter or reverse drive force to the rotor. Electronic braking requires some combination of complicated circuitry to control the reverse current flow, a more sophisticated and expensive motor or a reverse polarity power supply. Any of the foregoing requirements is costly. Another disadvantage of electronic braking is that it is complicated to design and implement and does not lend itself to single step applications. The fourth prior art technique is monitoring the position of the output member to control the drive signals fed to the motor so that backlash error is acceptably low. This technique is a complicated technique which is difficult to design and implement. It has the added disadvantage of not being suitable for applications requiring only single step moves at one time. All these disadvantages of the prior art have been overcome by this invention.
The advantages of this invention include significant improvement in controlling the rotational acceleration of a step motor rotor while minimizing the effects of backlash inherent in the mechanical system. Another advantage of this invention is that full current is applied during the final or single step move thus maximizing the positional accuracy of the rotor while also being adaptive to load changes. Another advantage of the invention is that it is particularly suited for applications requiring only a single step move of the rotor.
Another advantage of the invention is that the same consistency and level of accuracy is achieved as is obtained using complicated and expensive precision mechanical linkages and anti-backlash gears. Another advantage of this invention is that it provides a means for controlling the output member without monitoring the response of that output member. Another advantage of this invention is that it is particularly useful for systems requiring moves of a single step only yet is practical for driving the step motor at rated speeds while maintaining control over motor acceleration/deceleration.
Additional advantages of this invention are that complicated circuitry is not required, it is simple to design and implement and is flexible enough to be useful for numerous varied applications.